1. Field of the Invention
The present invention relates to chelation or sorption of transition metal ions by shellfish waste that has been partially treated but not fully converted to chitosan, a known chelating agent. The partially treated shellfish waste has been found to be a practical sorbent for removing metal ions from, e.g., wastewater.
2. Brief Description of the Background
Chitin, the second most abundant natural biopolymer (after cellulose), is a significant structural component in the shells of crustaceans (e.g., crabs, lobster and shrimp), in the exoskeletons of insects and in the cell walls of many microbes and higher fungi. Chitin is a polysaccharide consisting predominantly of unbranched chains of .beta.-(1,4)-2-acetamido-2-deoxy-D-glucose (also known as N-acetyl-D-glucosamine) residues. It may also be regarded as a derivative of cellulose, in which the C-2 hydroxyl groups have been replaced by acetamide residues, and it resembles cellulose in many of its properties. Its occurrence in lower animals and plants, and its isolation are described in the following references:
A. G Richards, The Integument of Arthropods (University of Minnesota Press, Minneapolis, 1951); F. von Wettstein, Handbuch der systematischen Botanik (F. Deuticke, Leipzig and Vienna, 4th ed., 1933); Hackman, Aust. J. Biol. Sci. 7, 168 (1954); Horowitz et al., J. Am. Chem. Soc. 79, 5046 (1957); Dweltz, Biochim. Biophys. Acta 44, 416 (1960); 51, 283 (1961); Carlstrom, ibid. 59, 361 (1962); Ramachadran, Ramakrishman, ibid. 63, 307 (1962); Foster, Webber, Advan. Carbohyd. Chem. 15, 371-393 (1960); C. Jeuniaux, "Chitinous Structures" in Comprehensive Biochemistry vol. 26c, M. Florkin, E. H. Stotz, Eds. (Elsevier, New York, 1971) pp. 595-632.
Chitin is an amorphous solid which is practically insoluble in water, dilute acids, dilute and concentrated alkalies, alcohol and other organic solvents. It is soluble in concentrated HCl, H.sub.2 SO.sub.4, 78-97% H.sub.3 PO.sub.4, and anhydrous HCOOH.
In the U.S. and most other countries, chitin is an essentially unutilized resource and even a significant waste problem for the shellfish industries. The amount of chitin potentially available from seafood wastes in the U.S. has been estimated at between 5000 to 8000 tons per year by Hattis and Murray (Industrial Prospects for Chitin From Seafood Wastes, MIT Seagrant Report No. 27, MIT, Cambridge, Massachusetts, August 1976).
It has been recognized that the product formed by deacetylating chitin has interesting and potentially useful properties. Although chitins may occur in nature in a slightly de-acetylated form, that which has been purposely and substantially de-acetylated is usually called chitosan. One property of chitosan is its ability to bind certain metal ions at least some of which binding may be by chelation. Chelation occurs through the free amino groups. To the inventor's knowledge, no careful study of chitosan has been performed to assess its potential as a practical, economically competitive material for use in ion exchange and chelation.
Although it serves as a good chelating agent, relatively pure chitosan is expensive to produce. Pure chitosan is produced by long and severe chemical treatment of a raw material such as crab shells. Purification of, e.g., shrimp or crab shells involves removing protein (often using enzymes) and minerals to leave behind the pure acetamido-polysaccharide; the latter material is then deacetylated by hydrolysis to produce the amino-polysaccharide chitosan. A typical prior art method for producing chitosan involves preliminary treatment of shellfish waste with a strong mineral acid (e.g., HCl) which decalcifies the shells, followed by treatment with strong caustic at elevated temperatures for about 24 hours to remove protein and all acetyl groups.
Common among most of the prior art methods to produce chitosan has been the desire to remove most or all of the acetyl groups from chitin, to result in a relatively pure chitosan, which is used as a chelating agent, among other uses.
Unlike these prior art methods, the present invention involves treating chitin in a relatively mild manner such that it is hydrolyzed to chitosan only partially and presumably only on the outer surfaces of the particles. The present materials are economical to produce since the treatment is quite mild, and yet, surprisingly, this partially treated material possesses chelating ability which is nearly equal to that of the more exhaustively treated material, and further, it exhibits enhanced susceptibility to elution of chelated metal ions. The intention underlying such mild treatment is to produce a lower-value-added sorbent from shellfish waste at lower cost.
Some of the prior art processes for treatment of chitin to result in a useful product are summarized hereinbelow. None of them discusses deacetylation of chitin to chitosan only on the surface of the particles as in the present invention.
Rigby, U.S. Pat. No. 2,040,879, is directed to substantially deacetylated chitin, the structure of which is nearly completely undegraded. According to Rigby et al, chitin is produced by boiling shell waste in 1% soda ash for 6 hours, washing, treating with 5% HCl until all salts have been removed, washing, boiling in 1% soda ash and detergent for 8 hours, and washing. The chitin so produced is then deacetylated to produce chitosan. The deacetylation step involves treatment with 40% sodium hydroxide at 110.degree. C. for 4 hours, followed by draining and washing. The process of Rigby has as one object reduced degradation of the backbone of the chitin or chitosan polymer.
Hung, Science Reports of the National Taiwan University, No. 7, pages 56-63, December, 1977, is directed to production of chitin and chitosan from shellfish waste by various treatments. Adsorbents for metal ions were prepared by treating the shells of 4 species of shrimp and 1 species of crab with various reagents, then grinding the shells into fine powders (100/200 mesh). The various pretreatments used were: A-80% ethanol for 24 hours, B-13.5% HCl for 24 hours, C-same as B but with additional treatment in 5% NaOH at 85.degree.-90.degree. C. for 135 minutes three times, D-same as A but with additional treatment in 5% NaOH at 85.degree.-90.degree. C. for 135 minutes. One major difference between this reference and the present invention occurs in the size of the particles involved. Hung involves very small particles, on the order of 75 to 100 .mu.m. The particles in the present invention are much larger since they are not ground as in Hung. The treatment process of chitin to produce chitosan is also different in Hung. The reference does not discuss a process which deacetylates chitin only on the surface.
Peniston et al, U.S. Pat. No. 3,553,940, is directed to a method of treating an aqueous medium with chitosan and derivatives of chitin to remove impurities. The treatment of chitin in this patent involves the following steps: shell waste is treated with HCl at room temperature for 2 to 3 hours to remove calcium carbonate, and is washed. Then the material is heated in 3% sodium hydroxide for 2 hours at 100.degree. C. and washed. The resulting product is then bleached with potassium permanganate followed by oxalic acid solution, and washed. Finally, the resulting material is treated with 40% sodium hydroxide at 150.degree. C. to remove some of the acetyl groups. The treatment time is not specified.
Peniston et al, U.S. Pat. No. 4,195,175, is directed to a process for the manufacture of chitosan in which chitin is treated with 35 to 50% sodium hydroxide solution, heated to 40.degree. to 80.degree., air is expelled, and the mixture is held at 40.degree. to 80.degree. C. for 160 to 40 hours. The product is then washed and dried to produce a chitosan product.
Peniston et al, U.S. Pat. No. 3,862,122, is directed to a method of producing chitosan from chitin, which involves deproteination of shell waste by countercurrent contacting with dilute sodium hydroxide (0.5-2%) at 50.degree. to 70.degree. C. for 1 to 4 hours. The material is then simultaneously decarbonated and deacetylated by countercurrently contacting with strong sodium hydroxide (30 to 50%) at 120.degree. to 150.degree. C. for about 1 hour.
Broussignac, Chemical Abstracts 69:59578x (1968), produces chitosan by treatment of chitin in dilute (5%) HCl for 24 hours, followed by deacetylation by treatment with 50% potassium hydroxide, 25% ethanol and ethylene glycol at 120.degree. C. for 16 hours.
Table 1 below summarizes the treatment regimens involved in some of the pertinent references mentioned herein. For comparative purposes, an example of a treatment method of the present invention is also given.
TABLE 1 ______________________________________ Condition and Amounts of Reagents Employed (per part of shell waste or of chitin) Step Deprotein- Deminer- Deacetyl- Reference ation alization ation ______________________________________ U.S. Pat. No. 4,195,175 -- -- 72 hr., 70.degree. C., 5 parts 50% NaOH per part purif. chitin U.S. Pat. No. 3,862,122 -- 80-90.degree. C., 120-150.degree. C., 30 min 23 1 hr., 10 parts parts 10% 40% NaOH sucrose per per part part depro- deproteinated teinated shells shells U.S. Pat. No. 3,533,940 100.degree. C., room 150.degree. C., 2 hr. 3% temp., 40% NaOH - NaOH - no 2-3 hr. N no inform- information HCl - no ation on time on amounts information or amounts on amounts U.S. Pat. No. 2,040,879 boil 6 hr., room 110.degree. C., Ex. 1 1% soda temp., 5% 40% NaOH - ash, boil HCl - no no inform- again 8 hr. information ation on time 1% soda on amounts or amounts ash + de- tergent U.S. Pat. No. 2,040,879 steam heat, room 115.degree. C., Ex. 2 4 hr. 8 temp., 6 hr. 48 parts parts 1/2% overnight, 40% NaOH soda ash, 3 8 parts 1% separate HCl treatments of 4 hrs. each Present method -- room 50% NaOH (Example) temp., 90.degree. C. 1 hr. 1 hr, 5% (results in HCl some de- proteinization also) ______________________________________
Additional patents and references which are related to the present invention are listed hereinbelow. None of them appears to disclose a process wherein chitin is deacetylated to chitosan only on the surface, as in the present invention.
Rigby, U.S. Pat. No. 2,072,771; PA1 Maltz, U.S. Pat. No. 4,436,731; PA1 Nagyvary, U.S. Pat. No. 4,363,801; PA1 Komiyama et al, U.S. Pat. No. 4,512,968; PA1 European Patent 0065491; PA1 British Patent 2,129,300; PA1 European Patent 0,028,126; PA1 British Patent 2,026,516; PA1 Chemical Abstracts 106:14375v; PA1 Chemical Abstracts 68:62641x; PA1 Chemical Abstracts 74:79292f; PA1 Chemical Abstracts 83:12563n; PA1 Chemical Abstracts 83:12707n; PA1 Chemical Abstracts 84:76100n; PA1 Chemical Abstracts 94:32563t; PA1 Chemical Abstracts 87:172265c; PA1 Chemical Abstracts 97:4611a; PA1 Chemical Abstracts 98:5675g; and PA1 Chemical Abstracts 95:25438y.
In spite of the above-described prior art, there has remained a need for new and more economical ways of chelating metal ions using shellfish-derived waste.